Trailhead Creating and using Custom Controllers Challenge Issue

I'm getting the error "The outputLink component is not binding to the ID of a case" for this challenge. I've checked several threads on here and I'm almost certain it should be correct. It does link to the details page for the case.

I initially just had the ID linking but have changed it so both case number and id link via id in case that mattered.

I've also previously tried having a complete url as the value but no luck.

But just to stress, its the Challenge check on the Trailhead that is failing with the message "The outputLink component is not binding to the ID of a case". The page loads fine for me and the links do redirect to the case details page.

I was having issue with the challenge not meet with errors complaining about the custom controller not working, even though a test case showed otherwise and the page rendered correctly. Ines's soltuion did work for me. Many thanks Ines for posting it.